Medical errors are common and dangerous. Approximately 20% of medical errors are related to prescription of medications. Most medication errors occur at transitions in care, such as when patients are admitted from ambulatory settings to hospitals or discharged from hospitals to ambulatory settings. Since prescriptions at hospital discharge are intended to be followed in ambulatory care, improving transitional and ambulatory care requires attention to the discharge prescriptions. With medication reconciliation (MR), multiple sources of medication information are reviewed, to determine which medications are currently active, and which medications should be prescribed. The process of MR is tedious and time-consuming. Although implementation of formal MR systems improves the fraction of cases undergoing MR, we know relatively little about the extent to which MR systems alter clinical outcomes. Research also indicates that MR should be integrated with computer-based provider order entry (CPOE), but this seldom if ever occurs. The specific aims of this study are to integrate an electronic MR system with an electronic prescribing system, conduct a randomized controlled trial of MR, and determine whether electronic facilitation of MR alters MR and the incidence of medication errors in ambulatory care. On a patient's hospital admission, a Web-based MR module will receive an automatically compiled outpatient medication list. Following discussion with the patient, medical personnel will update the list, which will then be delivered to the CPOE system and become actionable for prescribing. Main outcomes include adverse drug events and erroneous discrepancies between the pre-admission medication list and the medication list upon the patient's return to ambulatory care. We hypothesize that electronic facilitation of inpatient MR will improve completion of MR and will decrease the incidence of drug-related medical errors.